Even drinking a moderate amount of alcohol can cause brain damage over a prolonged period of time, scientists have said. In a study of more than 500 adults over 30 years, researchers found people who drank between 14 and 21 units per week were three times more likely to suffer from hippocampal atrophy—damage to the area of the brain involved in memory and spatial navigation.

The effect of heavy drinking on brain function has long been known. But what impact drinking within government guidelines has on a person’s brain health is not well understood.

In the latest study published in the BMJ, researchers from the University of Oxford and University College London looked at the impact of moderate alcohol consumption on the brain.

At present, U.K. guidelines say people should drink no more than 14 units of alcohol per week. In the U.S., women are advised not to drink more than 12.3 units, while for men it is up to 24.5 units a week. One unit is equivalent to a single, 25ml spirit measurement, a 250ml bottle of 4 percent beer or a 76ml serving of 13 percent wine.

The researchers looked at data on 550 healthy men and women who were not dependent on alcohol. On average, the participants were 43 at the start of the study. Their cognitive performance was measured regularly over 30 years and over this period, information on social factors was documented—including alcohol intake, smoking, medical history, education and physical activity.

After adjusting for these factors, researchers were able to work out what impact different levels of alcohol consumption had on a person’s brain health.

Their findings showed even drinking moderate amounts of alcohol—between 14 and 21 units—had a negative impact on cognitive function and harmful changes to the brain structure. People who drank 30 units per week were at most risk, moderate drinkers were far more likely to show signs of hippocampal atrophy than light and non-drinkers.

The researchers note this is an observational study, so they cannot draw any conclusions relating to cause and effect. They also say that all the data on alcohol use was self-reported and participants may have underestimated how much they drank. Data on some individuals was missing and they said they “cannot exclude the possibility we have included some people who were alcohol-dependent at some points during the study period.”

The study was also based on data of civil servants, meaning they tended to be white, middle class men. This means the findings are not necessarily representational of society.

However, they say the strengths of the study lie in the findings on long-term alcohol consumption together with the data relating to other lifestyle factors. "Alcohol might represent a modifiable risk factor for cognitive impairment, and primary prevention interventions targeted to later life could be too late," they wrote.

"Our findings support the recent reduction in UK safe limits and call into question the current U.S. guidelines, which suggest that up to 24.5 units a week is safe for men, as we found increased odds of hippocampal atrophy at just 14-21 units a week, and we found no support for a protective effect of light consumption on brain structure.”

In a related editorial, Killian A Welch, consultant neuropsychiatrist at the Royal Edinburgh Hospital, said: “[The] findings strengthen the argument that drinking habits many regard as normal have adverse consequences for health. This is important. We all use rationalizations to justify persistence with behaviors not in our long-term interest. With publication of this paper, justification of ‘moderate’ drinking on the grounds of brain health becomes a little harder.”